PHOTODISSOCIATION DYNAMICS OF THE PHENYL RADICAL VIA PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY

Author Institution: College of Chemistry, University of California, Berkeley, California 94720, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USAPhotofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl radical at 193 and 248 nm. Time of flight data collected for the C$_6$H$_4$, C$_4$H$_3$, and C$_2$H$_2$ photofragments show the presence of two decomposition channels. The only C$_6$H$_5$ decomposition channel observed at 248 nm corresponds to C ??H bond fission from the cyclic radical producing $ortho$-benzyne. The translational energy distribution peaks at 0 $kcal/mol$ and is consistent with no exit barrier for the H loss process. At 193 nm photodissociation, however, H loss was observed to be the minor channel, while the major decomposition pathway corresponds with decyclization of the C$_6$H$_5$ radical and subsequent fragmentation to $n$-C$_4$H$_3$ and C$_2$H$_2$. These two momentum matched photofragments have a translational energy distribution that peaks around 9 $kcal/mol$, indicative of a process that proceeds through a tighter transition state. Previous theoretical work on the unimolecular decomposition of the phenyl radical \textbf{1997}, 101, 6790.} predicts a second H loss process that occurs after C$_6$H$_5$ decyclization resulting in the linear C$_6$H$_4$ photofragment. This channel cannot be unambiguously discerned from the C$_6$H$_4$$^+$ time of flight data, but is believed to take place since decyclization is observed

Substrate Level Control of the Local Doping in Graphene

Graphene exfoliated onto muscovite mica is studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) techniques. Mica provides an interesting dielectric substrate interface to measure the properties of graphene due to the ultraflat nature of a cleaved mica surface and the surface electric dipoles it possesses. Flat regions of the mica surface show some surface modulation of the graphene topography (24 pm) due to topographic modulation of the mica surface and full conformation of the graphene to that surface. In addition to these ultraflat regions, plateaus of varying size having been found. A comparison of topographic images and STS measurements show that these plateaus are of two types: one with characteristics of water monolayer formation between the graphene and mica, and the other arising from potassium ions trapped at the interfacial region. Immediately above the water induced plateaus, graphene is insulated from charge doping, while p-type doping is observed in areas adjacent to these water nucleation points. However, above and in the neighborhood of interfacial potassium ions, only n-type doping is observed. Graphene regions above the potassium ions are more strongly n-doped than regions adjacent to these alkali atom plateaus. Furthermore, a direct correlation of these Fermi level shifts with topographic features is seen without the random charge carrier density modulation observed in other dielectric substrates. This suggests a possible route to nanoscopic control of the local electron and hole doping in graphene via specific substrate architecture